The present invention concerns polyolefin compositions comprising two polymer fractions with different Melt Flow Rate values, selected from propylene homopolymers and propylene-ethylene and/or other xcex1-olefin random copolymers, and a copolymer of ethylene with C4-C10 xcex1-olefins.
The compositions of the present invention present a unique balance of processability, mechanical properties and optical properties. In addition they present low/very low blush, reduced blooming and low content of fraction extractable in organic solvents. The said compositions can be easily processed by injection-molding and can be used for several applications, including housewares and toys, and in particular for food-contact applications.
Compositions comprising polypropylene and a rubbery phase formed by an elastomeric copolymer of ethylene with xcex1-olefins are already known in the art, and described in particular in European patents 170 255 and 373 660. Said compositions present impact resistance and, in the case of European patent 373 660, transparency values interesting for many applications, however the overall balance of properties is still not totally satisfactory in view of the high standards required by the market. Therefore there is a strong demand for compositions of this kind with improved properties. Such a goal has now been achieved by the polyolefin compositions of the present invention, comprising (percent by weight):
A) 60%-95%, preferably 70%-90%, more preferably 70%-88%, of a crystalline polypropylene component having a Melt Flow Rate (MFRA) value (measured at 230xc2x0 C. with 2.16 Kg load) of from 2.5 to 50, preferably from 5 to 50, more preferably from 10 to 30 g/10 min., and containing from 20% to 80%, preferably from 40% to 60%, of a fraction AI) having a Melt Flow Rate (MFRI) value (measured at 230xc2x0 C., with 2.16 Kg load) of from 0.5 to 8, preferably from 0.5 to 5, more preferably from 1 to 3 g/10 min., and from 20% to 80%, preferably from 40% to 60%, of a fraction AII);
B) 5%-40%, preferably 10%-30%, more preferably 12%-30%, of a copolymer of ethylene with one or more C4-C10 xcex1-olefin(s) containing from 10 to 40%, preferably from 15 to 30%, more preferably from 15 to 25%, of said C4-C10 xcex1-olefin(s);
said fractions AI) and AII) being independently selected from propylene homopolymers and random copolymers of propylene containing up to 15%, preferably up to 10%, of ethylene and/or C4-C10 xcex1-olefin(s); the ratio MFRA/MFRI being from 2 to 25, preferably from 4 to 20; the percentages of A) and B) being referred to the sum of A) and B), and the percentages of AI) and AII) being referred to the sum of AI) and AII).
From the above definitions it is evident that the term xe2x80x9ccopolymerxe2x80x9d includes polymers containing more than one kind of comonomers.
As previously said, the compositions of the present invention can be easily converted into various kinds of finished or semi-finished articles, in particular by using injection-molding techniques, as they possess relatively high values of MFR, associated with the said high balance of properties (in particular, of flexural modulus, impact resistance, ductile/brittle transition temperature, haze and gloss). The compositions of the present invention having values of MFR (230xc2x0 C., 2.16 Kg) of the overall composition equal to or higher than 4 g/10 min., in particular equal to or higher than 5 g/10 min., are preferred.
The value of MFR of fraction AII) (MFRII) can be easily determined, on the basis of the above said ranges of MFRI and MFRA values, by means of the known correlation between the MFR of a polyolefin composition and the MFR of the separate components, which, in the present case, can be expressed as follows:
ln MFRA=(WAI/WAI+WAII)xc3x97ln MFRI+(WAII/WAI+WAII)xc3x97ln MFRII
wherein WAI and WAII represent the weight of fractions AI) and AII) respectively.
Other preferred features for the compositions of the present invention are:
content of comonomer or comonomers in each of fractions AI) and AII) when at least one of them is selected from propylene copolymers: 0.5 to 15%, more preferably 0.5 to 10%, in particular 0.5 to 8% (0.5 to 5% when only ethylene is present, 1 to 10%, in particular 1 to 8%, when only C4-C10 xcex1-olefin(s) are present);
content of polymer insoluble in xylene at room temperature (23xc2x0 C.) (substantially equivalent to the Isotacticity Index) for fractions AI) and AII): not less than 80%, more preferably not less than 85%, in particular not less than 90%, for propylene copolymers; not less than 90%, more preferably not less than 95%, in particular not less than 97%, for propylene homopolymers, said percentages being by weight and referred to a single fraction;
Polydispersity Index (PI) for A): equal to or higher than 4, in particular from 4 to 12;
Intrinsic Viscosity [xcex7] of the fraction (of the overall composition) soluble in xylene at room temperature: 0.8 to 2.5 dl/g, more preferably, when high transparency is desired, 0.8 to 2, most preferably 0.8 to 1.9, in particular 0.8 to 1.5 dl/g.
The compositions of the present invention present at least one melt peak, determined by way of DSC (Differential Scanning Calorimetry), at a temperature higher than 140-145xc2x0 C. Component B) of the said compositions generally presents a melt peak, determined by way of DSC, at a temperature in the range from 120xc2x0 C. to 135xc2x0 C. Such melt peak, which is attributable to a polyethylenic-type crystallinity, is generally detectable in the DSC pattern of the overall composition, particularly when component A) is made of propylene homopolymers.
Moreover, the compositions of the present invention preferably have:
a Flexural Modulus of at least 700 MPa, in particular from 700 to 1300 MPa, when at least one of fractions AI) and AII) is selected from propylene copolymers, or of at least 1200 MPa, more preferably at least 1400 MPa, in particular from 1400 or 1500 to 2000 MPa, when component A) is made of propylene homopolymers (i.e., both AI) and AII) are propylene homopolymers);
Izod values at 23xc2x0 C. of at least 50 J/m, more preferably of at least 60 J/m in particular from 50 or 60 to 500 J/m;
tensile strength at yield: 15-38 MPa;
elongation at break: higher than 40%;
substantially no whitening (blush) when bending a plaque 1 mm thick;
fraction extractable in hexane (FDA 177, 1520): less than 10%, more preferably less than 9%, in particular less than 5.5% by weight;
fraction soluble in xylene at room temperature: less than 20%, more preferably less than 15%.
The Ductile/Brittle transition temperature and the optical properties (Haze and Gloss) are strongly dependent upon the Intrinsic Viscosity (I.V.) of the fraction (of the overall composition) soluble in xylene at room temperature.
The ductile/brittle transition temperature is lower the greater the said I.V. and is generally equal to or lower than xe2x88x922xc2x0 C., preferably equal to or lower than xe2x88x925xc2x0 C., more preferably equal to or lower than xe2x88x9210xc2x0 C., the lower limit being indicatively of about xe2x88x9260xc2x0 C.
Haze is lower the lower the said I.V. and is preferably lower than 30%, more preferably equal to or lower than 25%, for compositions wherein component A) is made of propylene homopolymers, equal to or lower than 20%, more preferably equal to or lower than 15%, for compositions wherein at least one of fractions AI) and AII) is selected from propylene copolymers. Said Haze values are measured on 1 mm thick plaques, prepared from nucleated compositions (in particular with dibenzylidene sorbitols).
Gloss is higher the lower the said I.V. and is preferably in the range from 30 to 150‰, more preferably from 40 to 130‰, measured in the same conditions as for Haze. It is therefore clear that in addition to the previously said preferred ranges of I.V. of the fraction soluble in xylene at room temperature, applicable when excellent optical properties are desired, another preferred range of said I.V. exists, namely from more than 1.5 to 2.5 dl/g, applicable when low ductile/brittle transition temperatures, and consequently improved impact resistance at low temperatures, are desired. In such range of I.V. the Haze values are generally in the range from 45 to 75%.
The compositions of the present invention are also characterized by reduced levels of blooming, as demonstrated by the fact that generally their Gloss values do not undergo a decrease of more than 30% under ageing (for instance after 9 days of ageing at 80xc2x0 C.).
The said C4-C10 xcex1-olefins, that are or may be present as comonomers in the components and fractions of the compositions of the present invention, are represented by the formula CH2xe2x95x90CHR, wherein R is an alkyl radical, linear or branched, with 2-8 carbon atoms or an aryl (in particular phenyl) radical.
Examples of said C4-C10 xcex1-olefins are 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene. Particularly preferred is 1-butene.
The compositions of the present invention can be prepared by sequential polymerization in at least three polymerization steps. Such polymerization is carried out in the presence of stereospecific Ziegler-Natta catalysts. An essential component of said catalysts is a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond, and an electron-donor compound, both supported on a magnesium halide in active form. Another essential component (co-catalyst) is an organoaluminum compound, such as an aluminum alkyl compound.
An external donor is optionally added.
The catalysts generally used in the process of the invention are capable of producing polypropylene with an Isotacticity Index greater than 90%, preferably greater than 95%. Moreover, said catalysts must have a sensitivity to molecular weight regulators (particularly hydrogen) high enough to produce polypropylene having MFR values from less than 1 g/10 min. to 100 g/10 min. or more.
Catalysts having the above mentioned characteristics are well known in the patent literature; particularly advantageous are the catalysts described in U.S. Pat. No. 4,399,054 and European patent 45977. Other examples can be found in U.S. Pat. No. 4,472,524.
The solid catalyst components used in said catalysts comprise, as electron-donors (internal donors), compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids. Particularly suitable electron-donor compounds are phthalic acid esters, such as diisobutyl, dioctyl, diphenyl and benzylbutyl phthalate.
Other electron-donors particularly suitable are 1,3-diethers of formula: 
wherein RI and RII are the same or different and are C1-C18 alkyl, C3-C18 cycloalkyl or C7-C18 aryl radicals; RIII and RIV are the same or different and are C1-C4 alkyl radicals; or are the 1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations. Ethers of this type are described in published European patent applications 361493 and 728769.
Representative examples of said dieters are 2-methyl-2-isopropyl-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane. 2-isopropyl-2-isoamyl-1,3-dimethoxypropane, 9,9-bis (methoxymethyl) fluorene.
The preparation of the above mentioned catalyst components is carried out according to various methods.
For example, a MgCl2.nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, is reacted with an excess of TiCl4 containing the electron-donor compound. The reaction temperature is generally from 80 to 120xc2x0 C. The solid is then isolated and reacted once more with TiCl4, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.
In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from 0.5 to 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is 5 to 20% by moles with respect to the magnesium dihalide.
The titanium compounds which can be used for the preparation of the solid catalyst component are the halides and the halogen alcoholates of titanium. Titanium tetrachloride is the preferred compound.
The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates.
The active form of magnesium halide in the solid catalyst component can be recognized by the fact that in the X-ray spectrum of the catalyst component the maximum intensity reflection appearing in the spectrum of the nonactivated magnesium halide (having a surface area smaller than 3 m2/g) is no longer present, but in its place there is a halo with the maximum intensity shifted with respect to the position of the maximum intensity reflection of the nonactivated magnesium dihalide, or by the fact that the maximum intensity reflection shows a width at half-peak at least 30% greater than the one of the maximum intensity reflection which appears in the spectrum of the nonactivated magnesium halide. The most active forms are those where the above mentioned halo appears in the X-ray spectrum of the solid catalyst component.
Among magnesium halides, the magnesium chloride is preferred. In the case of the most active forms of magnesium chloride, the X-ray spectrum of the solid catalyst component shows a halo instead of the reflection which in the spectrum of the nonactivated chloride appears at 2.56 xc3x85.
The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as Al-triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO4 or SO3 groups. The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.
The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Si- OR bond, where R is a hydrocarbon radical.
Examples of silicon compounds are (tert-butyl)2 Si (OCH3)2, (cyclohexyl) (methyl) Si (OCH3)2, (phenyl)2 Si (OCH3)2 and (cyclopentyl)2 Si (OCH3)2. 1,3-diethers having formulae described above can also be used advantageously. If the internal donor is one of these dieters, the external donors can be omitted.
As previously said, the polymerization process can be carried out in at least three sequential steps, wherein components A) and B) are prepared in separate subsequent steps, operating in each step, except the first step, in the presence of the polymer formed and the catalyst used in the preceding step. The catalyst is added only in the first step, however its activity is such that it is still active for all the subsequent steps.
Component A) is preferably prepared before component B).
In at least two (preferably consecutive) polymerization steps the relevant monomer(s) are polymerized to form fractions Al) and All) and in the other step(s) a mixture of ethylene and the C4-C10 xcex1-olefin(s) is polymerized to form component B). Preferably, fraction AI) is prepared before fraction AII).
The regulation of the molecular weight is carried out by using known regulators, hydrogen in particular.
By properly dosing the concentration of the molecular weight regulator in the relevant steps, the previously described MFR and [xcex7] values are obtained.
The whole polymerization process, which can be continuous or batch, is carried out following known techniques and operating in liquid phase, in the presence or not of inert diluent, or in gas phase, or by mixed liquid-gas techniques. It is preferable to carry out the polymerization in gas phase. However it is possible to carry out the propylene (co)polymerization steps using liquid propylene as diluent, and the other polymerization step(s) in gas phase. Generally there is no need for intermediate steps except for the degassing of unreacted monomers.
Reaction time, pressure and temperature relative to the two steps are not critical, however it is best if the temperature is from 20 to 100xc2x0 C. The pressure can be atmospheric or higher. The catalysts can be pre-contacted with small amounts of olefins (prepolymerization). The compositions of the present invention can also be obtained by preparing separately the said components A) and B) or even fractions AI), AII) and component B), by operating with the same catalysts and substantially under the same polymerization conditions as previously explained (except that a wholly sequential polymerization process will not be carried out, but the said components and fractions will be prepared in separate polymerization steps) and then mechanically blending said components and fractions in the molten or softened state. Conventional mixing apparatuses, like screw extrudres, in particular twin screw extruders, can be used.
The compositions of the present invention can also contain additives commonly employed in the art, such as antioxidants, light stabilizers, heat stabilizers, nucleating agents, colorants and fillers.
In particular, the addition of nucleating agents brings about a considerable improvement in important physical-mechanical properties, such as Flexural Modulus, Heat Distortion Temperature (HDT), tensile strength at yield and transparency.
Typical examples of nucleating agents are the p-tert.-butyl benzoate and the 1,3- and 2,4-dibenzylidenesorbitols.
The nucleating agents are preferably added to the compositions of the present invention in quantities ranging from 0.05 to 2% by weight, more preferably from 0.1 to 1% by weight with respect to the total weight.
The addition of inorganic fillers, such as talc, calcium carbonate and mineral fibers, also brings about an improvement to some mechanical properties, such as Flexural Modulus and HDT. Talc can also have a nucleating effect.
The particulars are given in the following examples, which are given to illustrate, without limiting, the present invention.